1. Field of the Invention
The present invention relates to an electronic ballast for controlling gas discharge lamps and, more particularly, to a frequency controlled ballast with power supply circuitry which limits in-rush current while assuring rapid, but soft ballast start and methods for controlling gas discharge lamps.
2. Description of the Prior Art
A gas discharge lamp, such as a fluorescent lamp, represents a complex load for a power source because the current drawn by the lamp is dependent upon the lamp's condition, conducting or non-conducting, and the electrical nature of the elements used in its accompanying ballast. In addition to the usual problems associated with controlling an electrical load, control of a fluorescent lamp or group of lamps is made more difficult by factors such as safety requirements, fluctuating line voltages, power factor requirements, electrically generated noise and efficiency considerations.
A gas discharge lamp presents a non-linear or variable load to a power source since the lamp requires that a minimum voltage across the lamp be reached before it will conduct and draw current through the lamp. "Instant start" ballasts and lamps rely on a high initial voltage (over 400 volts) to trigger discharge between unheated lamp electrodes. "Rapid start" ballasts and lamps rely on a low, preheating voltage supplied to the electrodes for approximately 2 seconds followed by a starting voltage that is typically between 200 and 300 volts which strikes the starting arc within the lamp. Regardless of its type, once the lamp turns on and conducts, the current it draws will rapidly increase unless a load is placed in series with the lamp to limit and stabilize the lamp current.
Early prior art true resistive ballasts provided the appropriate current limiting function, but consumed too much power and, as result, pulled down the ballast's operating efficiency. Magnetic or inductive based ballasts which followed improved overall efficiency somewhat, but the heavy weight and bulk of large inductors that were required because of the low input power frequency range of from 50 to 60 hertz, and the associated inductor core and copper losses, caused inductor based ballasts to lose favor and be overtaken by electronic ballasts.
Modern electronic ballasts offer improved operating efficiency and control functions not usually found in early ballasts. They typically provide a power conditioning circuit and a control circuit which insure sufficiently high voltage to initiate glow discharge within the lamps and an impedance to counteract the negative resistance properties of the glow discharge and filament heater current to promote rapid start. The power circuit usually includes a rectifying capability to change input alternating current (typically 50 to 60 Hertz and from 90 to 300 volts AC) to a form of direct current, a "boost" circuit to raise the inverted DC voltage levels to the operating levels needed by the lamps and an inverter function to convert the rectified direct current to alternating current at higher frequencies (typically 20 to 60 kilohertz which reduces the size of the inductors used). Very often, the boost circuitry, which can be of the driven, self-triggering or oscillating type, includes a power factor or conditioning circuit that attempts to maintain the operational load presented by the ballast to the power line as close to unity (a pure resistive load) as operating conditions permit.
U.S. Pat. No. 5,461,287 to Russell et al, entitled BOOSTER DRIVEN INVERTER BALLAST EMPLOYING THE OUTPUT FROM THE INVERTER TO TRIGGER THE BOOSTER, is an example of a modern electronic ballast wherein a triggered boost circuit, a driven inverter and a low voltage signal generator are used to power the lamp control circuit. In the event of a fault, the operation of the signal generator is interrupted thereby shutting off the boost circuit and the inverter.
Another prior art ballast example of this type will be found in U.S. Pat. No. 5,591,289 to Katyl et al, entitled ELECTRONIC BALLAST WITH LAMP CURRENT CORRECTION CIRCUIT, wherein a frequency dependent, regulated power supply, a power oscillator/driver circuit, power factor correction circuit and a feedback circuit are combined in an electronic ballast to maintain constant current in the ballast regardless of the number of lamps controlled by the ballast. The power circuit correction circuit is turned on through a resistor connected to the power supply.
U.S. Pat. No. 5,471,118 TO Nilssen, entitled ELECTRONIC BALLAST WITH POWER-FACTOR-CORRECTING PRE-CONVERTER, is directed to a ballast that features an FET implemented full wave bridge rectifier combined with a boost circuit and power factor correction pre-converter.
Another electronic ballast described in U.S. Pat. No. 5,416,387 to Cuk et al, entitled SINGLE-STAGE, HIGH POWER FACTOR, GAS DISCHARGE LAMP BALLAST, seeks to increase ballast efficiency and reliability while reducing size and cost by utilizing circuitry that combines a ballast's usual power conversion stages into a single stage. One feature of this arrangement is its claimed suitability for both instant and rapid start type lamps.
One of the common characteristics of prior art electronic ballasts is that their starting operational requirements are hard on the lamps and cause shortened life as well as other operating deficiencies, such as, for example, wasted power, too much or too little light output or inefficient power usage. In addition, prior art dimming electronic ballasts employed pulse width modulation techniques that required power consuming additional circuitry to "clean" up resulting voltage ripples caused by the pulse width modulation.
Moreover, many available prior art electronic ballasts with power factor correction circuits, like some of those identified above, cannot be guaranteed to start on the first haversine produced by their rectifier circuits. This delay in turn-on of the ballast, of necessity, adds delay to the time it takes to start the lamps it controls, even for so-called rapid start or instant-on lamps.
It is, therefore, a primary object of the present invention to provide an electronic ballast for gas discharge lamps that is dependent upon frequency control.
It is also an object of the present invention to provide an electronic ballast that started as quickly as possible, specifically on the first half-cycle of power available to it.
It is also an object of the present invention to have such a quick starting ballast function in cooperation with its power factor compensation circuitry while still limiting in-rush current.
A further object of the present invention is to provide initial input power to a ballast's power factor compensation or correcting circuitry and thereafter maintain such input power in an efficient manner, preferable by removing the in-rush current circuit from the ballast's operational path after starting, when it is not needed, thereby avoiding the loss of power it would otherwise consume.
It would also be an object of the present invention to provide lamp functions, such as dimming and fast lamp turn-on, in a universal ballast that handled all of the foregoing requirements correctly under the different ranges of input voltage and frequency provided in different regions of the world thereby yielding manufacturing economies of scale, lowering cost and reducing inventory requirements for vendors and distributors of electronic ballasts. Thus, modern lamp control circuitry or electronic ballasts, are presented with a myriad number of functional requirements and operating conditions that have resulted in various prior art solutions.
Still another object of the present invention is to accomplish the foregoing over the worldwide range of expected voltages and frequencies without interfering with the control aspects of the ballast in which it is employed.